1. Field of the Invention
The present invention relates to a method of manufacturing silicon carbide (SiC) single crystals in which micropipe defects are closed and silicon carbide single crystals with closed micropipe defects manufactured by the method.
2. Description of the Related Art
When an SiC single crystal is produced by the modified Lely method (sublimation method) using an SiC single crystal as a seed crystal, hollow tubes called micropipe defects with a diameter ranging from sub-microns (.mu.m) to several microns (.mu.m) are extended approximately along the growth direction, and contained in a grown crystal. An SiC single crystal having micropipe defects is not suitable as a substrate for electronic device formation since the micropipe defect significantly degrades the electric property of the device. Therefore, reduction of the micropipe defects is an important task for producing the SiC single crystal.
The methods for reducing the micropipe defects have been proposed in U.S. Pat. No. 5,679,153 and laid-open Japanese Patent Publication No. 5-262599(Japanese Patent No. 2804860).
In the method disclosed in U.S. Pat. No. 5,679,153, an epitaxial layer having reduced micropipe defects (defect density: 0 to 50 cm.sup.-2) is allowed to grow on a SiC substrate having micropipe defects (defect density: 50 to 400 cm.sup.-2) utilizing a phenomenon that micropipe defects are closed in the epitaxial layer on the SiC substrate by a liquid phase epitaxy from a melt of SiC in silicon.
In the method described in laid-open Japanese Patent Publication No. 5-262599(Japanese Patent No. 2804860), a single crystal revealing no hexagonal etch pit at all in alkali etching, that is, a single crystal having no micropipe defect is grown on the seed crystal by using a plane vertical to (0001) plane as a growing plane of the seed crystal.
In any of the above-described two methods, the single crystal is newly grown on the seed crystal and micropipe defects in the growing layer are reduced.
In the former method, an epitaxial layer having a thickness of 20 to 75 .mu.m has to be grown by the liquid phase epitaxy technique for obtaining portions containing no micropipe defects, and micropipe defects are still existing below in this thickness range. Further, if the single crystal is grown again by a sublimation method using the above-formed epitaxial layer as a seed crystal, there is a possibility that sublimation from the closed portions of micropipe defects generates micropipe defect openings again since the closed portions of micropipe defects are thin. Therefore it is difficult to prepare a seed crystal and to suitably regulate sublimation growing condition to present in the closed portions from sublimating.
Meanwhile, although the latter method is effective for inhibiting the micropipe defect generation, stacking faults are newly generated in the grown single crystal. The substrates with stacking faults are known to exhibit an anisotropy of electron transport. Therefore, the crystal is not suitable for the substrate for the electronic device.